The interactions between Ca2+ ions and sphingomyelin play crucial roles in a wide range of cellular activities. However, little is known about the molecular details of the interactions at interfaces. In this work, we investigated the interactions between Ca2+ ions and egg sphingomyelin (ESM) Langmuir monolayers at the air/water interface by subwavenumber high-resolution broadband sum frequency generation vibrational spectroscopy (HR-BB-SFG-VS). We show that Ca2+ ions can induce ordering of the acyl chains in the ESM monolayer. An analysis of the one alkyl-chain-deuterated ESM revealed that the Ca2+ ions do not affect the N-linked saturated fatty acid chain, although they make the sphingosine backbone become ordered. Further analysis of the SFG-VS spectra shows that the interactions between ESM and Ca2+ ions make the orientation of the methyl group at the end of sphingosine backbone change from pointing downward to pointing upward. Moreover, a large blue shift of the phosphate group at the CaCl2 solution interface indicates, to our knowledge, new cation binding modes. Such binding causes the phosphate moiety to dehydrate, resulting in the conformation change of the phosphate moiety. Based on these results, we propose the molecular mechanism that Ca2+ ions can bind to the phosphate group and subsequently destroy the intramolecular hydrogen bond between the 3-hydroxyl group and the phosphate oxygen, which results in an ordering change of the sphingosine backbone. These findings illustrate the potential application of HR-BB-SFG-VS to investigate lipid-cation interactions and the calcium channel modulated by lipid domain formation through slight structural changes in the membrane lipid. It will also shed light on the interactions of complex molecules at surfaces and interfaces.

The salt effects on molecular orientation at air/liquid methanol interface were investigated by the polarization-dependent sum frequency generation vibrational spectroscopy (SFG-VS). We clarified that the average tilting angle of the methyl group to be θ = 30° ± 5° at the air/pure methanol surface assuming a δ-function orientational distribution. Upon the addition of 3 mol/L NaI, the methyl group tilts further away from the surface normal with a new θ = 41° ± 3°. This orientational change does not explain the enhancement of the SFG-VS intensities when adding NaI, implying the number density of the methanol molecules with a net polar ordering in the surface region also changed with the NaI concentrations. These spectroscopic findings shed new light on the salt effects on the surfaces structures of the polar organic solutions. It was also shown that the accurate determination of the bulk refractive indices and Raman depolarization ratios for different salt concentrations is crucial to quantitatively interpret the SFG-VS data.The average tilting angle of the methyl group at the air/liquid methanol interface was found to change by more than 10° when 3 mol/L NaI was added into the liquid bulk.

Photochromic molecules often exhibit switchable hyperpolarizabilities upon photoisomerization between two molecular states and can be widely applied in nonlinear optical materials. Photoisomerization can occur through either one-photon or two-photon processes. Two-photon-induced isomerization has several advantages over one-photon process but has not been fully explored. In the present study, we have used second harmonic generation to investigate the two-photon-induced isomerization between spiropyran and merocyanine at the air/water interface. We show that spiropyran and merocyanine can be converted into each other reversibly with 780-nm laser-beam irradiation through two-photon processes. We also investigated the isomerization rates under various incident laser powers. Quantitative analysis revealed that the isomerization rates of spiropyran and merocyanine depend differently on the laser power. We attribute the difference to the distinct molecular structures of spiropyran and merocyanine. At the interface, nonplanar spiropyran molecules exist mainly as monomers, whereas planar merocyanine molecules form aggregates. Upon aggregation, steric hindrance effects and excitonic coupling efficiently arrest the photoisomerization of merocyanine. This work provides an in-depth understanding of two-photon-induced isomerization at the interface, which is beneficial for designing and controlling optical thin-film materials.

Sum frequency generation spectroscopy (SFG) has been widely used to study the interfacial chemistry of aqueous salt solutions of biological or environmental importance. Most of the SFG data analysis used the same bulk refractive index for different salt concentrations despite of the variations of the refractive indices. Here we systematically investigate the influence of the refractive index on the SFG intensities at various experimental conditions. It is discovered that the SFG intensities are the most sensitive to the refractive index at solid/liquid interfaces nearby the total internal reflection geometries. At air/liquid interfaces, the effect of the refractive indices is also nonegligible. Consequently some important SFG results, such as the response of water structures to the ionic strength at the SiO2/aqueous interfaces, are necessary to be reevaluated. These conclusions on the effect of the small variations of the refractive index are generally useful for the common practice of SFG data analysis.A small change of the bulk refractive indices with the elevating salt concentrations was found to significantly affect the sum frequency generation intensities.

With the development of the nonlinear optical technique such as SHG (second harmonic generation), the in situ measurements of the chirality in the monolayers at the air/water interface have become possible. However, when performing the SHG measurement of the chirality in a monolayer, it is still a great challenge to obtain the chiral signals with a good S/N (signal-to-noise) ratio. In this Letter, interfacial assemblies with induced supramolecular chirality were used to amplify the weak chiral SHG signals from the monolayers at the air/water interface. Tetrakis(4-sulfonatophenyl) porphyrin (TPPS) J aggregates were used as the subphase, and when chiral amphiphilic molecules were spread on it, chiral domains of the amphiphile/TPPS J aggregates were formed and then significantly amplified chiral signals that otherwise could not be detected. Moreover, the sign of the DCE (degree of chiral excess) changed with the chirality of the amphiphilic molecules, thus providing a possible way to obtain the absolute chiral information in situ in the monolayers.

Understanding and control of the surface properties such as molecular orientations are of great importance in numerous applications of ionic liquids. However, there remain discrepancies among the previous experimental and theoretical studies on the surface orientation and structures of room temperature ionic liquids (RTIL) systems. In this article, the orientation of 1-butyl-3-methylimidazolium ([bmin]) cation at the air/liquid interface of a characteristic RTIL, 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), was investigated by the sum frequency generation vibrational spectroscopy (SFG-VS). Detailed polarization and experimental configuration analyses of the SFG-VS spectra showed the possibility of a small spectral splitting in the CH3 symmetric stretching region, which can be further attributed to the probable existence of multiple orientations for the interfacial [bmim] cations. In addition, the (N)-CH3 vibrations were absent, ruling out the prediction by several recent molecular dynamics simulations which state that portions of the [bmim] cations orient with a standing-up (N)-CH3 group at the ionic liquid surface. Hence, new realistic theoretical models have to be developed to reflect the complex nature of the ionic liquid surface.

Chiral porphyrin assemblies are promising molecular materials because they possess unique biological compatibility and excellent electronic properties. Metal ions can strongly affect the formation of supramolecular chirality. In this paper, we investigated the effect of metal ions in the subphase on the supramolecular chirality of a porphyrin derivative with two long hydrophobic chains (TPPA2a) at the air/aqueous interfaces by means of second harmonic generation linear dichroism (SHG-LD). It was found that TPPA2a can form chiral assemblies at the air/aqueous interface even though the molecule itself is achiral. Furthermore, metal ions added into the subphase have a considerable effect on the interfacial supramolecular chirality: Zn2+ inhibits the formation of supramolecular chirality, while Cu2+ promotes the formation. We suggest that the effect of metal ions on the supramolecular chirality is due to the coordination between the metal ions and TPPA2a molecules. To clarify the coordination mechanism, we also performed UV–vis measurements of TPPA2a Langmuir–Blodgett (LB) films and SHG-LD experiments on TPPA4, which is similar to TPPA2a but without ester groups. These results revealed that the metal ions did not interact with the central nitrogen of porphyrin rings, while the coordination between metal ions and the ester groups possibly affects the supramolecular chirality. This is a novel mechanism involving coordination between metal cations and side chains of porphyrin derivatives, and it may provide a deeper understanding of the supramolecular chirality of porphyrin assemblies.

In this investigation, the adsorption of benzonitrile at the air/water interface was addressed using vibrational sum-frequency spectroscopy. Using ppp and ssp polarization combinations, the authors detected the symmetric stretching mode of the cyano (CN) group and calculated the orientation of benzonitrile at the interface. In addition, the adsorption isotherm was determined in terms of the hyperpolarizability element by varying the bulk benzonitrile concentration. The adsorption energy was obtained from fitting this isotherm. This work will add to our understanding of chemical processes relevant to retention, degradation, and photolysis of benzonitriles in the environment.

Nonequilibrium adsorption and subsequent reorientation of organic molecules at electrode/electrolyte interfaces are important steps in electrochemical reactions and other interfacial processes, yet real-time quantitative characterization and monitoring of these processes, particularly for the reorientation step, are still challenging experimentally. Herein, we investigated the nonequilibrium adsorption process of 4-(4-(diethylamino)styryl)-N-methyl-pyridinium iodide (D289) molecules from acetonitrile solution onto a polycrystalline platinum electrode surface using real-time second harmonic generation (SHG) in combination with the potential step method. The time-dependent SHG curves exhibit two distinct regimes, which were interpreted with a two-step adsorption model consisting of a fast adsorption and a slow reorientation step for D289 on the surface. D289 was assumed to initially adsorb in an orientation perpendicular to the surface and then reorient to a parallel orientation. We derived a quantitative mathematical expression containing a biexponential function to fit the temporal SHG curves and obtain the rate constants for the two steps. The rate constants for fast adsorption and the slower reorientation processes show similar potential-dependent behavior: the rate decreases with an increase in the negative potential. We further proposed a molecular mechanism involving the displacement of D289 and CH3CN molecules adsorbed on the electrode interface to explain this potential-dependent behavior. On the basis of such analysis, we obtained a detailed picture of the adsorption of D289 molecules on the Pt electrode/CH3CN electrolyte, which consists of three consecutive steps: diffusion, adsorption, and reorientation. The results of this study may shed light on adsorption mechanisms at the electrode/electrolyte interface as well as at biological and other functional material interfaces.

α-Dipalmitoylphosphatidylcholine (DPPC) Langmuir monolayers at the air/water interface are an important biomimetic system. Previous studies using optical imaging have shown that enantiomerically pure DPPC molecules form periodical homochiral domains with close packing. However, it is unclear whether the chirality of these domains is stable or dynamic. Recently, we investigated the chirality of l-DPPC Langmuir monolayers using second-harmonic-generation linear dichroism (SHG-LD). The change in the degree of chiral excess (DCE) and its sign with time indicated that the chirality transformed from homochiral to heterochiral domains, with an associated change in handedness. Here we propose that hydrolysis of l-DPPC is responsible for the formation of the opposite chiral state. On the basis of quantum chemistry calculations and effective pair potential (EPP) theory, we give a molecular explanation for this transformation. These results provide an insight into the lipid monolayers that the chiral state of domains composed of chiral lipids is dynamic.

The effect of pH on the conformation of surface alkyl groups and the structure of interfacial water molecules on poly[2-(dimethylamino)ethyl methacrylate] (PDEM) at the air/water interface were investigated with sum frequency generation vibrational spectroscopy (SFG-VS). At pH 4.1, the hydrogen bonding SFG spectra were similar to that of the air/pure water interface. As the pH increased from 5.4 to 9.6, the SFG intensities of both highly ordered hydrogen bonding (3200 cm−1 band) and less-ordered hydrogen bonding (3400 cm−1 band) were enhanced because of the charge-induced effect of deprotonated PDEM. The free OH peak disappeared completely because it was replaced by interfacial PDEM molecules. At pH 11.5, a new spectral band appeared at about 3580 cm−1 in the ppp and sps spectra, and this could be assigned to the C2v asymmetric stretching mode of the water molecules through molecular symmetry simulation. These hydrogen bonding structures are fully consistent with the conformational change of PDEM alkyl groups, and PDEM molecules act as a Hofmeister solvent. PDEM molecules are kosmotropic when they are charged and become more chaotropic as the pH increases.